This invention relates to bearings, and more particularly, to bearings between parts made of wood or equivalent materials subject to swell and shrink due to changes in atmospheric conditions. The invention has particular utility in connection with the pivotal connections in a piano action, that is, the centers on which the action members swing back and forth, and will be described and illustrated in that context.
A piano action consists of a series of levers, usually made of wood, which are pinned together at certain hinge points known as "centers". These connections are usually effected with tongue-and-fork arrangement held together by means of a lateral or transverse pin. These pins are generally of "German silver" with considerable nickel content so that they will not corrode, and are manufactured with precise tolerances on diameter and concentricity. The pin is held firmly in the central or tongue member, as the hole is drilled for an interference fit. The fork member turns relative to the pin and is provided with a bushing with a view toward achieving a noiseless, efficient and durable action.
The bushing has for many years been made of bushing cloth, a special all-wool felted fabric especially designed and manufactured for this purpose. It has resilience and softness sufficient to eliminate noises, and to absorb impacts in order to eliminate failure of the action parts through fracture and to maintain accurate alignment of the parts. However, this material is hygroscopic, tending to swell in moist atmospheric conditions and to shrink in dry conditions. Consequently, under moist conditions the center often becomes so tight as to interfere with the functioning of the pivotal connection and the corresponding piano key either loses all speed and sensitivity of action or fails to function entirely. A common "fix" for malfunctions due to tightening of the center because of high humidity is the application of a drop of light oil, or an alcohol and water mix, which usually relieves the pivot only temporarily in that it tends to tighten up again with continued exposure to moist conditions. Conversely, with dry conditions the centers occasionally become too loose, resulting in rattles in the action and inaccuracy in the alignment of parts, with consequent loss of power and control in the so-called "touch qualities" of the action.
Other shortcomings of the use of bushing cloth are the inconvenience involved in inserting it in the small holes in the fork members, the necessity for glueing or otherwise securing it to the fork members, and the rather frequent requirement that they be re-glued when, after a period of use, the original glue dries out and the bushing tends to work itself out of the hole in the fork arm. Also, the glue has a tendency to penetrate through the bushing cloth, causing noisy centers.
These costly disadvantages have been eliminated to a considerable extent by providing between the mating action parts a one-piece bushing of suitable plastic or elastomeric material, such as a fluorocarbon resin of the type marketed under the name "Teflon, " as described in U.S. Pat. No. 3,240,095, assigned to the assignee of the present application. The bushing described therein is provided with an integral annular flange at one end, which is positioned on the internal or tongue side of each arm of the fork member. The bushing is inserted from the inside into a bushing hole of correct size to afford a light push fit so as to eliminate any possible distortion of the internal diameter. The flange prevents any possible movement of the bushing outwardly, and, with proper fit, no movement whatsoever, either rotary or inwardly toward the tongue member can occur. The design of the bushing eliminates the necessity for glueing it in place thereby removing the risk of loose and consequently noisy and inaccurate bushings, and of glue-soaked bushings and of bushings uneven in their action because of uneven distribution of glue. The flange also stiffens the bushing and also acts as a spacer to provide the necessary clearance between the mating parts.
Although the bushing described in the aforementioned patent performs admirably when the bushing holes in the fork arms are accurately aligned, the mating parts accurately sized, and the bushing pin and the internal bore of the bushing are sized relative to each other to provide the proper interference fit, and indeed, these conditions have been achieved sufficiently well that bushings of the design described in the patent have been used in preference to bushing cloth for about ten years in the pianos manufactured by applicant's assignee. However, this long-term experience with the patented bushing has demonstrated that it is not without fault. For example, in spite of observance of usual care in the fabrication of the wooden parts of the piano action, the drilled holes in the two fork arms are not perfectly aligned with each other, or may be misaligned with the drilled pin-receiving hole in the tongue part, with the consequence that the internal bores of the two bushings, when inserted with a push fit in the drilled holes in the fork arms, are likewise misaligned with the bearing pin, thus causing the pin to bind in the bushing and not rotate with the desired ease. This problem is compounded by the sizing of the bushings relative to the size of the holes in the fork arms to afford a light push fit, so as to eliminate any possible distortion of the internal diameter of the bushing and to increase the effective length of the contact between the bearing pin and the internal bore of the bushing. Also, it has been found that, contrary to the thinking expressed in the patent that the clearance between mating parts could be reduced to zero, or even operated under compression (by virtue of the low friction of Teflon), it has been found in practice that when the clearance is reduced to this degree the center does not work freely when the wood parts are distorted by extreme conditions of moisture and/or temperature. It has been found, also, that when the bushings are manufactured to a tolerance slightly less than 0.001 inch, as taught in the patent, variations in diameter between bushings become sufficiently significant that sizing or reaming, both during manufacture and in the field, is often necessary to cause the center to work with the correct freedom; obviously, this contributes to the cost of manufacture of the piano as well as to the cost of servicing and maintenance.
It is an object of this invention to eliminate these costly disadvantages of the currently used one-piece Teflon bushing through the provision between the mating action parts of an improved one-piece bushing.